Joinable story characters?

Accepted Answer

This is a bit SPOILER filled, so read at your own risk.

In order to unlock the story characters as joinable party members, you must first beat the main story. Once you have unlocked the post game and gotten your shiny transport back, you can attempt these DLC quests to unlock party members:

[Taken from Zaraf's Quest Guide here on GameFAQs]

Quest #125 - Perk Up, Patty!Location: Stornway; Talk to Erinn from behind the counter in the innRequest: Cheer Patty up.Solution: Talk to Patty from behind the counter then Zoom to Batsureg and talk to the chief's father, Batkhaan. Afterwards, go back to Stornway and talk to Patty again. At night, talk to the ghost in Batsureg and SAIL to Khalaag coast which is north of Batsureg. You can't use the Starflight Express to get there or you will not get the scene. Examine the gravestone to the east to receive the Friendant. Report back to Patty.Reward: Patty joins your partyPre-req: DLC, Finish Quest 123Repeat: NoNotes: Story; Available for download on Jul 30, 2010.

Quest #139 - Has Sellma Sold Out?Location: Stornway; Talk to Erinn from behind the counter in the innRequest: Help Sellma.Solution: Talk to Sellma, then the banker from behind the counter. You should get the Guestbook. Now talk to Erinn and Sellma and leave for Gleeba. At Gleeba go into the Plumbed Depths and go through the magic door. Make your way to the graveyard and access the secret stairs to B3 to fight Rover (#279). Report back to Erinn.Reward: Sellma joins your partPre-req: DLC, Finish Quest 125Repeat: NoNotes: Story; Available for download on Sep 17, 2010.

Quest #157 - Inn for a Penny, Inn for an InnyLocation: Stornway; Talk to Erinn from behind the counter in the innRequest: Defeat Octagoon (#219) 20 times.Solution: Talk to the king after initating quest. Fight Octagoon in mid to high level Ruin grottos. Report to the king once complete and then to Erinn.Reward: Erinn joins your partyPre-req: DLC, Finish Quest 156, access to mid to high level Ruin grottosRepeat: NoNotes: Story, available for download on Jan 2, 2011.

Quest #163 - When a Star Wishes Upon YouLocation: Stornway; Talk to Pavo from behind the counter at the innRequest: Defeat Excalipurr (#292) for the Prayer Pearl.Solution: He is a grotto boss that can be found in a high level grotto and report back to Pavo once completed. Then go to the Realm of the Almighty and speak with Celestria. Talk to Pavo again and go to Angel Falls and talk to Aquila. Go back and talk to Pavo one last time.Reward: Aquila joins your partyPre-req: DLC, Finish Quest 157, access to high level grottoRepeat: NoNotes: Story; Available for download on or around Feb 11, 2011.